The invention is related to apparatus and methods for monitoring pneumatic limb compression therapy given to the limbs of human subjects in order to help prevent deep vein thrombosis, pulmonary embolism and death.
Limb compression systems of the prior art apply and release pressure on a patient""s extremity to augment venous blood flow and help prevent deep vein thrombosis (DVT), pulmonary embolism (PE) and death. Limb compression systems of the prior art typically include: a source of pressurized gas; one or more pneumatic sleeves for attaching to one or both of the lower limbs of a patient; and an instrument connected to the source of pressurized gas and connected to the sleeves by means of pneumatic tubing, for controlling the inflation and deflation of the sleeves and their periods of inflation and deflation. In U.S. Pat. No. 3,892,229 Taylor et al. describe an early example of one general type of limb compression system of the prior art known as an intermittent limb compression system; such systems apply pressure intermittently to each limb by inflating and deflating a single-bladder sleeve attached to the limb. In U.S. Pat. No. 4,013,069 Hasty describes an example of a second general type of limb compression system of the prior art, known as a sequential limb compression system; such systems apply pressure sequentially along the length of the limb by means of a multiple-bladder sleeve or multiple sleeves attached to the same limb which are inflated and deflated at different times. Certain intermittent and sequential limb compression systems of the prior art are designed to inflate and deflate sleeves thereby producing pressure waveforms to be applied to both limbs either simultaneously or alternately, while others are designed to produce pressure waveforms for application to one limb only.
One major concern with all pneumatic limb compression systems of the prior art is that the therapy actually delivered by these systems may vary substantially from the expected compression therapy. For example, a recent clinical study designed by one of the inventors of the present invention, and involving the most commonly used sequential pneumatic limb compression systems of the prior art, showed that the pneumatic limb compression therapy actually delivered to 49 patients following elective total hip replacement surgery varied widely from therapy expected by the operating surgeons in respect of key parameters of the therapy shown in the clinical literature to affect patient outcomes related to the incidence of deep venous thrombosis, pulmonary embolism and death. These key parameters included the rates of pressure rise delivered by each of the inflatable bladders of the sleeves and the maximum pressures delivered by each of the inflatable bladders. The study methodology involved continuous monitoring of the pressure of the compressed air in the pneumatic sleeves of these systems, permitting the pneumatic compression therapy actually delivered to patients to be directly monitored throughout the prescribed period of therapy and compared to the expectations of operating surgeons. The results of this clinical study indicated that the expected therapy was not delivered to any of the 49 patients monitored: therapy was only delivered an average of 77.8 percent of the time during the expected periods of therapy; the longest interruptions of therapy in individual subjects averaged 9.3 hr; and during 99.9 percent of the expected therapy times for all 49 patients monitored in the study, values of key outcomes-related parameters of the therapy actually delivered to the patients varied by more than 10 percent from expected values. The unanticipated range of variations that was found in this clinical study between expected and delivered pneumatic compression therapy, within individual patients and across all patients, may be an important source of variations in patient outcomes in respect of the incidence of deep vein thrombosis, pulmonary embolism and death, and may be an important confounding variable in comparatively evaluating reports of those patient outcomes. The present invention addresses many of the limitations of prior-art systems that have led to such unanticipated and wide variations between the expected therapy and the therapy actually delivered to patients.
Limb compression systems currently available do not have the capability of accurately producing a desired pressure waveform in combination with sleeves having differing designs and varying pneumatic volumes, or when sleeve application techniques vary and the resulting sleeve snugness varies, or when sleeves are applied to limbs of differing sizes, shapes and tissue characteristics. Such variables produce substantial variations between the expected and actual pressure waveforms delivered by limb compression systems. Clinical staff using such prior-art systems have very inaccurate and limited knowledge of what pressure waveforms have actually being applied to the patient relative to what was prescribed. Clinical staff using such systems also have no knowledge of the time intervals between occurrences when the expected therapy matches the therapy actually delivered. These are significant limitations with systems of the prior art, as evidence in the clinical literature suggests that applied pressure waveforms having different shapes and waveform parameters produce substantially different changes to venous blood flow and that both the duration of compression therapy and interruptions in compression therapy have an effect on the incidence of DVT, embolism and death.
Some limb compression systems of the prior art attempt to record and display the total cumulative time during which pneumatic compression therapy was delivered to a patients limb, but do not differentiate between times when the delivered therapy was near the expected therapy and when it was not. For example, commercially available systems such as the Plexipulse intermittent pneumatic compression device (NuTech, San Antonio Tex.) and Aircast intermittent pneumatic compression device (Aircast Inc., Summit, N.J.) record the cumulative time that compressed air was delivered to each compression sleeve. These are typical of prior-art systems which include simple timers that record merely the cumulative time that the systems were in operation.
In U.S. Pat. No. 5,443,440 Tumey et al. describe a pneumatic limb compression system capable of recording compliance data by creating and storing the time, date and duration of each use of the system for subsequent transmission to a physician""s computer. The compliance information recorded by this system contains only information relating to when the system was used on a patient and the cumulative duration of usage. Tumey et al. cannot and does not record or monitor times when pressure-related values of the delivered therapy matched the expected therapy and when they did not.
A major limitation of Tumey et al. and other limb compression systems of the prior art is that key parameters of pneumatic compression therapy that are known to affect patient outcomes are not monitored and recorded. This is a serious limitation because evidence in the clinical literature shows that variations in applied pressure waveforms produce substantial variations in venous blood flow, and that delays and interruptions in the delivery of pneumatic compression therapy affect the incidence of DVT. One key parameter identified by the inventors of the present invention is the interval between successive occurrences of delivered pressure waveforms having expected values of certain waveform parameters known to affect patient outcomes. Because this key parameter is not monitored as therapy is delivered by prior-art systems, variations between delivered and expected therapy cannot be detected as they occur, and clinical staff and patients cannot be alerted to take corrective measures for improving therapy and patient outcomes.
Because prior-art systems do not monitor the interval between successive occurrences of delivered pressure waveforms having expected values of certain waveform parameters known to affect patient outcomes, and because such prior-art systems do not therefore have alarms to alert clinicians and patents that a maximum time interval has elapsed during which the expected therapy was not delivered to the patient, then the operator and the patient cannot adapt such systems during therapy, including for example sleeve re-application, sleeve repositioning or changing certain operating parameters of the instrument supplying pressurized gas to the sleeve, to help assure that the prescribed and expected therapy is actually delivered to the patient throughout as much as possible of the prescribed duration of therapy.
Additionally, limb compression systems do not subsequently produce the recorded values of key outcomes-related parameters for use by physicians and others in determining the extent to which the prescribed and expected pressure waveforms were actually applied to the patient for use by third-party payors in reimbursing for therapy actually provided, and for use in improving patient outcomes by reducing variations in parameters known to produce variations in patient outcomes.
The present invention provides apparatus and method for monitoring the application of a varying pressure to a patients limb from a sleeve positioned on the limb in order to help augment the flow of venous blood in the limb and thereby reduce the incidence of deep vein thrombosis, pulmonary embolism and death. More specifically, the present invention includes: transducing means for producing a sleeve pressure signal indicative of pressure applied by the sleeve to the limb; waveform parameter measurement means responsive to the sleeve pressure signal for measuring the value of a predetermined pressure waveform parameter and for producing a waveform parameter signal indicative of the measured value of the waveform parameter; and interval determination means responsive to the waveform parameter signal for producing an interval signal indicative of an interval between a first occurrence when the measured value of the waveform parameter is near a predetermined parameter level and the next occurrence when the measured value of the waveform parameter is near the predetermined parameter level.
The present invention includes means to allow an operator to select the predetermined pressure waveform parameter and the predetermined parameter level from a plurality of predefined parameters and parameter levels. In the present invention, the pressure waveform parameter can be a predetermined variation in the estimated level of pressure of gas in the sleeve that augments the flow of venous blood into the limb proximal to the sleeve from the limb beneath the sleeve.
The interval determination means of the present invention can include means for measuring a number of intervals during therapy, each corresponding to the time between an occurrence when the measured value of the waveform parameter is near the predetermined parameter level and the next occurrence when the measured value of the waveform parameter is near the predetermined parameter level. The interval determination means can further include a clock for determining the clock times when occurrences are measured.
Alarm means are included in the present invention for producing an indication perceptible to the operator and the patient when a measured interval exceeds a predetermined maximum interval, thereby allowing the operator or the patient or the operator to take corrective action in an effort to reduce future measured intervals to values below the predetermined maximum interval.
In the present invention, if the sleeve is pneumatic and applies pressure to the limb when inflated with pressurized gas from a pressurizing means, the pressure transducing means may be connectable to the sleeve through tubing means so that it communicates pneumatically with the sleeve and only communicates pneumatically with the pressurizing means through the sleeve.